Here's the challenge: I, a level 17 Bard taught by the College of Compulsion (a homebrew subclass), am hired by some random bumpkins to go and get rid of the Tarrasque all by my lonesome, for 5 silver pieces. I accept and trek my way over there.

Once there I cast Phantasmal Force at the beast as many times as I need to in order to wear down its 3 legendary Resistances - up to 16 times - whilst leading him to a nearby beach.

After all of its resistances are blown down the drain I pull a final Hail Mary and cast True Polymorph upon the foul beast. Let us say that my d12 bardic inspiration was enough to transform him due to his failing his saving throw.

I turn the horrible beast into a chest-sized stone boulder, light enough for me to carry without too much hassle.

I take him to the waters of the beach I've been leading him to during our scuffles together. Having hired a friend to wait by the beach with a quick ship in hand, we set off for the deepest part of the ocean, the unnamed trench. There I drop the chest sized rock into the ocean to sink to the bottom of the sea. After which my friend and I book it the hell out of there, like there were devils at our heels.

Will the Tarrasque suffocate underwater and die?

No where in all of the Monster Manuals I've studied does it say that the Tarrasque can breathe underwater, or swim. In particular the 2nd edition monster Manual said, it could not be chopped up normally otherwise it would just regenerate.

I thus ask, as a related sequel to the basic question: if the beast was slain without so much as making it bleed, can it ever truly hope to regenerate, and can I truly claim those 5 silver pieces, or should I just get my wizard friend to wish it away with children's dreams?

Clarification: I am using 5th edition, but I've seen that people all across multiple forums use the Tarrasque's regenerative ability from 2nd edition. I am unsure whether or not that ability was still considered canon in the Tarrasque's lore: I felt that I needed to include that as part of the problem to solve.

This question ultimately boils down to this: Can the Tarrasque (even if it has Regeneration) drown?

Yes*

*: Assuming absolutely everything goes precisely as you expect it to.

From the Basic Rules...here's how Suffocation (i.e. Drowning, in this case) works.

A creature can hold its breath for a number of minutes equal to 1 + its Constitution modifier (minimum of 30 seconds).

When a creature runs out of breath or is choking, it can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

For example, a creature with a Constitution of 14 can hold its breath for 3 minutes. If it starts suffocating, it has 2 rounds to reach air before it drops to 0 hit points.

Source

"Is dying and cannot regain hit points" is pretty clear. Regeneration doesn't work when you are drowning, because you cannot regain hit points.

We can assume that suffocation works on the Tarrasque because of the 'there are no secret rules' standard...and there are plenty of creatures in D&D that specifically call out that they don't need to breathe (see: Undead), or that they can breathe water (see: green dragons). So, since the Tarrasque doesn't have such a feature, we can assume it needs air to survive.

So, the next question is, how deep does the trench have to be to drown the Tarrasque?

Well, the D&D 5E rules generally assume that any creature can swim, but...

While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed.

Source

So, the Tarrasque has a Speed of 40 feet. It does not have a Swimming Speed, so it can move at 20' while swimming. It has a Constitution modifier of +10, so we know it can comfortably hold its breath for 11 minutes (The rules make no assumption about whether or not you got a good breath before going under, so a DM may tweak this in some cases). Once it runs out of breath, it can survive for an additional 10 rounds before it starts dying.

So, given that a single Round lasts for 6 seconds, and thus 10 rounds is 1 minute, the Tarrasque can survive without air for 120 rounds.

Assuming it correctly ascertains which way is 'up' and starts swimming as hard as it can, it will cover 40' per round (20' movement, 20' dash) in normal movement then, assuming it can burn all of its Legendary Actions to move, that's an extra 30 feet per round. Thus, in the 120 rounds it has before it begins dying, it will be able to cover 8,400 feet.

Thus, you must find a body of water more than 8,400 feet deep to drop the Tarrasque-boulder into. This is....not too difficult. The average depth of the Pacific Ocean is about 12,100 feet deep...and the deepest part of the Mariana Trench (Challenger Deep) is about 36,070 feet deep.

So, assuming your plan goes precisely as you expect...the Tarrasque wastes its Legendary Resistances on your Phantasmal Force spells, and doesn't just tank the damage/effects (no DM that I know would waste an LR on something so petty). Assuming it follows you where you want it to go (instead of, y'know, eating the town). Assuming it doesn't eat you in the intervening time (A Tarrasque at a dead run can cover up to 140' per turn on ground [Dash and Legendaries], and its attacks have up to a 20' reach...so you'll need to take Significant Measures to stay ahead of it). Assuming it fails its save against True Polymorph. And assuming it didn't destroy your boat. Then yes...you can drown the Tarrasque if you drop it somewhere deep enough.

Addendum

This assumes you release True Polymorph before it runs its full duration. If you do not, then the spell is 'permanent' and the Tarrasque does not drown, because it is still a rock. The spell going 'permanent' seems pretty likely, as I doubt you'd be able to get the Tarrasque-boulder out to the middle of the ocean before 1 hour is up. If the spell is ever dispelled without removing it from the bottom of the ocean...then it'll drown.

It's possible, but requires a lot of unlikely events.

It definitely can drown

There is nothing in the stat block that states the Tarrasque can breathe underwater, so it can't.

If it can be put in water deep enough that it can't get out in time before suffocation, then it will drown.

In addition, the 5e stat block also does not include regeneration as an ability. So this version will not regenerate and you don't need to consider that in this exercise.

But there a lot of holes in your methodology

You are making a lot of assumptions that are unlikely to actually work.

1. The tarrasque may choose not to use it's legendary resistance on something that doesn't cause it much damage (phantasmal force)


2. The tarrasque may not follow you. In fact, why would it?


3. Your bardic inspiration will reduce it's saving throw roll by at most 12. They still have a +5 to the save and advantage. It isn't guaranteed by any means that they'll fail the throw.


4. You are also assuming that as you get in range to cast all those spells that the Tarrasque will just sit there and take it. With a move of 40, and a legendary action move of 20, and a reach of up to 20 on the tail...odds are you're gonna get hit.

It also may not matter. Tarrasque as rock falls, it dies.

If you just keep the tarrasque under true polymorph for the full duration, it's a rock. At the bottom of the ocean.